After Effects Tutorials

Usually when you’re keying blue or green screen you really only want one particular subject or object on screen and the rest of the background is redundant. So you usually create a rough matte or mask around that subject restricting the area to only where the action that you want is taking place. We call this a garbage matte. This garbage matte also limits the area of background key color (blue/ green) and allows for a more precise key.

You can do this manually, but a really quick and precise way to achieve a great garbage matte is using Auto Trace function.

- Bring in your footage and ensure it is selected.
- Select Effect/ Keying/ Keylight (this gives us a basic key)
- Select Layer/ Auto-Trace
- A dialogue box should appear
- Make sure “Preview” checkbox is on (this way you can see you results on your footage before applying)
- Check the radio button for “Work Area” so it applies to the whole duration and not just the current frame.
- Set the “Channel” drop down menu to “Alpha”
- Keep “Tolerance” at 1 pixel
- Keep “Threshold” at 50% or adjusted to suit your footage
- Keep “Minimum Area” to 10 pixels
- Keep Corner Roundness to 50%
- Check the box “Apply to new layer”
- Select “OK” and it should start to process (may take a minute)
- This should have now created a new layer with a close mask to the footage underneath

- Now we want to expand this mask/ matte a little
- Select the Auto-Trace solid
- Select Effect/ Matte/ Simple Choker
- In Effect Control window - Set Choke Matte between -10 to -20, enough to give a bit of room around the edge of the footage
- In the Timeline window - Set your footage layer TrackMatte drop down menu to “Alpha Matte”
- This tells the layer to use a transparency reference from the above layer

- That’s pretty much it. You may want to play with the settings in Keylight to pull a closer key, but it should require a lot less tweaking than if you didn’t already pull this great garbage matte.